One countermeasure in occupant protection during a vehicular crash is the seat-belt restraint system. The force imparted on the chest, or other body part, of an occupant by the seat belt during a crash depends on various factors, such as the vehicle speed and deceleration, as well as occupant size. Safety technology associated with seat belts often includes pretensioners and load-limiting devices to manage energy appropriately in a timely manner to minimize the injury to the vehicle occupants.
Often vehicular seat-belt retractors include features that limit force applied against a seat occupant and that adapt belt tightening. These features can adjust based on various factors such as vehicle speed, occupant size, seating position, lapsed time, and the like during or prior to an impact. For example, many active devices and technologies used with seat belts rely on various sensors that provide instantaneous data such as vehicle speed, vehicle deceleration, occupant weight, occupant position, seat position among other parameters as designed to activate the seat belt safety system during a crash. Some systems have continuously changing mechanisms based on data during normal driving and pre-crash sensing so that the seat belt safety system is optimized to the driving condition of the vehicle as well as occupant.
The devices that control seat-belt functionality add cost and weight and are primarily used for the vehicle front-row occupants due to the proximity to stiffer structures of the vehicle such as the steering wheel, dashboard, glove box, instrument panel, etc. Energy absorption of the front-seat occupants during a crash is critical due to possibility of secondary impact of occupants on stiffer interior components.
Rear-seat occupants usually face the back of front seats, which can be designed to have a lesser risk of injury as compared with the stiffer interior components often positioned in the front of the vehicle. As such, although rear seat restraint systems might employ force-liming features that include near real-time data gathering sensors, other mechanism might also be used that are less complex.